A sealing film for a sealing layer is produced, for example, by way of blown film extrusion or flat film extrusion. The film laminate for producing the bag is generally created by laminating (which is to say joining by way of an adhesive layer) multiple films. When producing sealing films from blown polyethylene (PE) (blown film) or cast polypropylene (PP) (cast film), according to the present state of the art so called slip additives (lubricants) or antiblock additives are added. The task of these is to render the usually relatively tacky polyolefins (such as PE or PP) smoother, so that these, during further processing, are able to slide better across the metal surfaces of the packaging machines or against themselves. If this step is not taken, undesirable machine stoppages and/or wrinkled sealed seams or leaking packagings may arise.
Processing such film laminates in packaging machines typically requires coefficients of friction (COF) of the sealing layer against steel in the range of 0.15 to 0.30, and of the sealing layer against itself in the range of 0.2 to 0.4. In particular, when processing the film laminates into bags, known as flow packs, in form-fill-seal (FFS) machines, the coefficient of friction against steel is a crucial quality feature of a packaging laminate.
The coefficients of friction indicated in the present application may be ascertained by way of the following test specification:
On a test block measuring 66×60×16 mm and having a weight of 500 g, a sample of a crease- and wrinkle-free sealing film is clamped onto one side of the test block (66×60 mm). The surface of the film to be tested must, of course, face outwardly. For clamping, the sample of the film may be larger than the size of the side of the test block. So as to measure the coefficient of friction against steel, the test block is placed on a steel table with the side on which the film is clamped. The test block is then pulled across the steel table, and the force required to do so is measured. The coefficient of friction is then ascertained as a ratio of the measured force and the weight of the test block (500 g). The procedure for measuring the coefficient of friction of the sealing layer against itself is the same, except that a crease- and wrinkle-free film is likewise clamped (with the side to be tested facing outside) onto the test table, onto which the test block is placed. Using a tensile testing machine, the test block is pulled over a measuring distance of 50 mm across the surface at a constant speed of 150 mm/min, and the tensile force is measured.
Usually, a distinction is made between the coefficient of static friction, which is derived from the maximum force before the test block moves, and the coefficient of dynamic friction. The latter is derived from the substantially constant, average force during the constant, jerk-free movement of the test block. Excessively tacky films move only in a jumpy way and thus cannot be measured since the forces fluctuate too drastically. Such films are unusable in a practical setting.
To achieve these coefficients of friction, according to the prior art slip additive concentrations having an S-value of 16,000 to 25,000 are used in the sealing film. The S-value is defined as the product from the layer thickness of the sealing film and the content of slip additive in ppm (parts per million).
Typically, slip additives used are oleamides, or the now preferred erucamides (ESA), which migrate from the sealing film outwardly over time and deposit on the surface of the sealing film, where they act as a lubricating film. The greatest disadvantage of these products is that these slip additives migrate, which can give rise to the following disadvantages:                The sliding friction of the PE or PP sealing film changes with increasing temperatures as a result of the improved solubility of the slip additives in the PE or PP, whereby the processing conditions of a film laminate comprising such a sealing film as the sealing layer change. This can make processing such film laminates (in a packaging machine) or such sealing films (in a laminating process) significantly more difficult.        The sliding friction changes after the film laminate has been laminated due to migration of the slip additives from the sealing film into the adhesive and/or laminating partner, whereby, once again, the processing conditions can change. This can make processing such film laminates significantly more difficult.        The laminating partner of the sealing film, such as PET or BOPP, becomes smoother due to the uptake of the slip additive. This may result in the film laminate no longer being transportable in the packing machine, whereby further processing would be impossible.        
Antiblock additives are usually mineral fillers (such as silicates or talcum), the addition of which increases the surface roughness of the sealing film. While antiblock agents do not tend to migrate, use of these alone however does not sufficiently lower the coefficient of friction (COF) of the sealing film, and thus the sliding properties. While pure PE has a COF of 0.5 to more than 1 (complete blocking), minimal coefficients of friction of 0.3 against steel can be generated when using only antiblock additives. However, this is only possible if the added concentration is high, and the transparency of the resulting sealing film is thereby reduced, which is generally undesirable. To achieve the desired COF, it was therefore considered necessary to add slip additives.
When producing packagings in the form of bags, a film laminate, as described above, is often folded to yield a bag and is fused or sealed. The film is typically a multi-layer laminate, for example composed of a transparent outer layer, such as made of biaxially oriented polyethylene terephthalate (BOPET) or biaxially oriented polypropylene (BOPP), an inner sealing layer made of a sealable polymer in the form of a sealing film as described above, such as made of polyethylene (PE) or polypropylene (PP), and an optionally interposed barrier layer, such as made of aluminum or metallized plastic material (such as metallized PET). Sealing or fusing, as is sufficiently known, typically takes place between temperature-controlled sealing jaws, which are pressed together, whereby the sealing layer of the film melts and establishes the joint during subsequent cooling. Thus, in the present connection, sealable shall be understood to mean that the melting temperature of the sealing layer makes sealing possible. A wide variety of materials are used for the sealing layer, which are meltable and compressible at typical sealing temperatures above 100° C. This requirement results in various mixtures and co-extrudates of low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ethylene vinyl acetate (EVA) and similar materials. Folding of the film, however, causes varying material thicknesses in the overlapping region, which during sealing can result in incomplete sealed seams, whereby the created bag forms undesirable air channels, for example.
This is schematically illustrated in FIG. 1 based on the example of a bag 1, which is a vertical tubular bag here. Here, the film of the bag 1 is first folded lengthwise to yield a tube and sealed along the longitudinal seam 2. At the upper and lower ends of the bag 1, the tube is sealed by way of a respective cross seam 3 so as to form a bag 1, whereby the product present therein is enclosed in the bag 1. The overlapping region of the two sealed seams, which is to say between the longitudinal seam 2 and the cross seam 3, is illustrated in enlarged form in FIG. 1. Due to the varying material thicknesses along the cross seam 3, it is possible that the overlapping film 5, in particular in the region of overlapping sealed seams, cannot be fully compressed by the sealing jaws 9a, 9b, whereby, during sealing of the cross seam 3, an air channel 4 may form in this region, causing the bag to leak. The film 5 is designed as a three-layer laminate here, comprising an outer BOPET layer 6, an intermediate layer 7 made of aluminum, and an inner sealing layer 8 made of PP. Similar problems also occur with other bag types, such as cross-bottom bags, stand-up bags, block bottom bags and the like, in the overlapping region of multiple film layers.
Similar problems can also occur when sealing so-called lidding films (generally composed of an aluminum base layer and a sealing layer applied thereon) on the edge of plastic containers, as is common in yogurt packagings, for example. Such lidding films are generally made of aluminum, plastic or paper, onto which a sealing layer is applied. Due to manufacturing tolerances during the production of the plastic containers and/or during the production of the film laminates of the lids, differences in thickness may also arise here, which cannot be compensated for during sealing by the pressure of the sealing jaws, and which can result in leaking of the packaging.
So as to reduce this problem during sealing, special materials have already been developed for use as the sealing layer; however, these are relatively expensive, and the packaging industry is thus reluctant to use these.
The thickness of the sealing layer cannot be reduced since the sealing layer must have a certain degree of compressibility. To be able to make the sealing layer thinner, special polymers are often admixed to the material of the sealing layer, which in turn, however, make the material more expensive again.
EP 2 537 770 A1 describes a film material comprising a foamed polymer layer, in particular for the production of bags for granular packaged goods. As a result of the foamed polymer layer, it is to be achieved that the contour of the granular packaged goods is not apparent on the outer bag surface.
US 2011/0293204 A1 describes a foamed, compressible polymer layer as a sealing layer to improve the sealing characteristics.
US 2005/0247960 A1, in turn, describes a film comprising an embossed sealing layer for forming a bag for vacuum packaging, wherein the embossing forms gaps, which form air channels during vacuum packaging through which air can be better removed. A visible pattern, such as in the form of letters or an arbitrary shape, can be provided as the embossing. To ensure that the embossing is easy to see with the naked eye and to ensure the function as an air channel during vacuum packaging, the embossing must be relatively deep, and in general significantly deeper than 100 μm. The formed air channels must be >˜1 mm wide to allow a reasonable volume flow for removal of the air from the packaging to be achieved.
Embossed sealing layers are also used to prevent covering lids from adhering to one another when stacked on top of one another, which can cause problems during processing in processing machines. The embossing creates an air cushion between individual adjoining covering lids, whereby the covering lids can be easily and reliably separated. Examples of this can be found in EP 2 149 447 A1 or WO 2006/096894 A1.